1. Currently, 100% inspections are conducted on flexible printed circuit boards (FPC). Naturally, in addition to checking for FPC disconnections and short circuits using specialized equipment, numerous other items require visual inspection. General traces can be inspected manually or with a magnifying glass magnified 2 to 3 times; however, a high-power microscope is necessary for inspecting high-density traces. Traces approximately 100μm in size should be inspected using a magnifying glass with 5-10 times magnification, while those measuring 50-100μm should be examined with 10-20 times magnification. For traces smaller than 50μm, a stereo microscope with a magnification of 20 times or more is recommended. It’s important to note that higher magnification does not always equate to better results. To ensure efficient inspections, a wide field of view is also crucial. Even with high magnification, inspection efficiency cannot be enhanced without the aid of electronic image magnification functions.

2. Double-sided FPC

The use of Automatic Optical Inspection (AOI) instruments to detect defects in flexible printed circuit boards is still primarily a part of mass production. Although AOI has been implemented in the tape reel process, it is limited to inspecting circuit defects and can only partially substitute for human inspectors. Flexible circuits differ from standard digital circuits, requiring special programming for effective inspection. Adapting to the rapid advancements in miniaturized circuits remains a challenge.

Here’s the revised version of your article:

1. As circuit density increases, the magnification of the microscope used for inspection will also rise, leading to longer inspection times per unit area. The amount of man-hours needed for inspecting flexible printed boards is significant. With continued advancements in circuit density, this proportion will only grow. Although a reduction in defect rates could improve inspection speed, the ongoing increase in circuit density means that the pass rate for precision pattern circuits is unlikely to see substantial gains.

2. Not all inspection items for flexible printed circuit boards are conducted at the final stage, particularly defects related to circuits and coverage, which are better identified during the manufacturing process. While in-process inspections cannot fully replace final inspections, they do play a role in enhancing overall production efficiency.

3. FPC reinforced board processing involves using film-like adhesives, which are typically protected by release films on both sides. An adhesive film with one side covered is attached to the reinforcement board, followed by shape and hole processing, and then laminated with the flexible printed board using a hot roll lamination method. Depending on the materials used, the dimensional accuracy may vary. Rigid boards made from epoxy glass cloth or paper-based phenolic laminates can be processed with CNC drilling and milling machines or molds. Polyester and polyimide films can also be shaped using knife molds for simpler designs. Generally, the film-like reinforcing plates do not require fine hole processing, allowing for CNC drilling and mold use. If processing can be automated efficiently, manufacturing costs may be reduced.

4. Aligning and positioning the reinforcement board onto the flexible printed circuit board with the processed shape and holes is challenging to automate and constitutes a significant portion of processing costs. This operation must be performed manually, and if a flexible printed board requires multiple reinforced boards made from different materials, costs increase. Conversely, simple designs or the use of user-friendly jigs can significantly enhance production efficiency and reduce costs. All PCB factories are striving to improve this process, yet skilled personnel are still essential for operation.

5. There are two types of bonding for reinforced boards: pressure-sensitive (PSA) and thermosetting adhesives, each requiring different levels of labor for processing. Using the pressure-sensitive type is straightforward; simply peel off the release film, align it with the flexible printed board, and apply pressure in a short time, even using just hand pressure. For required bonding strength, a simple press may be applied for a few seconds or through a hot press roll.

6. The use of thermosetting adhesives is more complex, generally requiring a pressure of 3–5 MPa (30–50 kg/cm²) and high temperatures of 160–180 degrees Celsius, maintained for 30–60 minutes. To avoid stressing the flexible printed board, uniform pressure on the reinforced board is crucial; uneven pressure may lead to breaking at the ends of the reinforced board.

7. Additionally, double-sided adhesive films with release films on both sides can bond flexible printed boards, or flexible printed boards to rigid ones, effectively creating a composite akin to half a rigid printed board. The processing and curing steps for this method are similar to those used in the laminating process for reinforced boards.
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me